1. Field of the Invention
The present invention relates to a high luminance inorganic phosphor powder having excellent light emission characteristics that is suitable for forming a phosphor layer in a plasma display panel (PDP), a fluorescent lamp, a fluorescent display tube, a solid state light emitting diode (LED), or the like, a method of manufacturing the phosphor powder, and a phosphor composition containing the phosphor powder.
2. Description of the Prior Art
To form a film-like phosphor layer in a display device such as a PDP, a fluorescent lamp, a fluorescent display tube, or the like, a phosphor green sheet or a phosphor ink or phosphor paste comprising a phosphor powder of an oxide type, a sulfide type or the like dispersed in a resin and/or a solvent is stuck or applied onto a glass substrate, and then baking is carried out. Moreover, with a solid state light emitting diode such as a white LED, a phosphor layer is formed on the light emitting surface of the device by curing a resin composition comprising such a phosphor powder dispersed therein.
As the above-mentioned phosphor powder, a powder comprising relatively large irregularly shaped or spherical particles of particle size approximately 3 to 10 μm is generally used.
However, in the case of using such a phosphor powder in a fluorescent lamp or any of various display devices using short wavelength ultraviolet radiation or a low-speed electron beam as an excitation source, the penetration of the ultraviolet radiation or electron beam into the particles is low, and hence excitation cannot be carried out as far as the inside of the phosphor particles. The region contributing to light emission is thus limited to a surface layer of the phosphor particles, and hence the efficiency is poor, leading to high cost.
Moreover, in the case of using a phosphor powder having a large particle size, to increase the coverage of the phosphor layer, the thickness of the phosphor layer must be made high. As a result, the amount of the phosphor applied increases, and hence the cost increases. Furthermore, due to the thickness of the phosphor layer being high, absorption and scattering of the excitation ultraviolet radiation and absorption and scattering of the visible light emitted through the fluorescence increase, and hence with a transmission type light emitting apparatus such as a fluorescent lamp, the light emission efficiency of the apparatus drops.
To resolve these problems, it is thought to be desirable to make the phosphor powder as fine as possible so as to reduce the thickness of the phosphor layer without reducing the coverage of the phosphor layer. However, with conventionally obtained fine phosphor powders, upon making the phosphor powder finer the luminance of light emission drops, and hence the actual state of affairs is that a fine phosphor powder having a sufficient luminance of light emission for practical use has not been obtained.
Specifically, most phosphor powders currently put to practical use are manufactured through the flux method which is a solid-phase reaction method. In this method, raw material powders are mixed together and heated at a high temperature with a flux in a baking vessel such as a crucible to bring about a solid-phase reaction, and then the reaction product is ground in a ball mill or the like, whereby the phosphor powder is manufactured. With this method, to increase the compositional homogeneity, heat treatment must be carried out for a long time at a high temperature, but through the heat treatment the product undergoes much grain growth and thus becomes lumpy, and hence to obtain a fine phosphor powder, grinding must be carried out for a long time with a very large force. In this case, due to being subjected to physical shock and chemical reaction during the grinding, the particle surface undergoes alteration, and moreover many defects are produced on the surface of and inside the powder, and hence the luminance of light emission drops drastically. It has thus been almost impossible in practice to obtain a phosphor having a particle size of not greater than 2 μm and a luminance sufficient for practical use.
On the other hand, a method of manufacturing an inorganic phosphor having a mean primary particle size of not more than 200 nm and a higher-order aggregate particle size of not more than 1.0 μm using a sol-gel method which is a wet method has also been disclosed (see Japanese Patent Publication No. 2001-303045A). However, with the sol-gel method, the cost is high for reasons such as high-purity raw materials being required. Moreover, through heat treatment at a high temperature which is carried out to remove hydroxyl groups from and crystallize the powder produced, the particles produced are prone to being sintered more than necessary and thus becoming coarse. To make the particles fine, as with the solid-phase reaction method, it is thus necessary to carry out grinding with a very large force, and hence many defects are produced on the surface of and inside the powder, and thus the luminance of light emission drops.
The manufacture of solid spherical fine particles using a spray pyrolysis method is also known (see Japanese Patent Publication No. 2000-336353A), but the ability to carry out mass production is poor, and moreover it has again been difficult to obtain a high luminance phosphor powder having a mean particle size of not greater than 2.0 μm. It is thought that this is because with the spray pyrolysis method, the residence time of the particles in the heating apparatus is short at approximately 0.1 to a few tens of seconds, and hence the crystal lattice of the phosphor particles produced contains many defects and much strain caused by rapid cooling after the heating. If the phosphor particles obtained through the pyrolysis method are subjected to annealing treatment at a high temperature to increase the luminance, then the particles agglomerate. However, in the case of treating at a temperature at which agglomeration does not occur, a sufficient luminance is not obtained. To reduce the effects of the rapid cooling, trials have been carried out in which the residence time during the spray pyrolysis is made to be several minutes or more, but a drop in the manufacturing efficiency is unavoidable, and hence this method is not practical industrially.
Moreover, to obtain the same effects as in the case of making the phosphor particles fine, the use of flake-like phosphor particles has been proposed (see Japanese Patent Publication No. 11-144625A). Such flake-like phosphor particles are manufactured using a solid-phase reaction method, with a plate-shaped phosphor powder being obtained by shortening the time of the solid-phase reaction. However, with the phosphor particles obtained through this method, only the surface portion has high crystallinity, and hence if the wavelength of the excitation light is longer than that of the 147 nm vacuum ultraviolet radiation used in the PDP, then sufficient luminance of light emission is not obtained.